It is known that the so-called submarine phenomenon may occur in an impact situation such as a vehicle crash by the vehicle occupant slipping forward under the seat belt. This is prone to occur when the occupant sits in the front end of the seat or when the back rest is tilted rearward, which reduces the effectiveness of the seat belt in restraining the vehicle occupant or prevents the desired parts of the vehicle occupant from being restrained.
Therefore, it is conceivable to raise the front end of the seat by providing a projection in a front end of the seat frame, or by installing a panel in a front end of the seat frame. However, the submarine effect may not be prevented if the projection or the panel is too low, and the comfort of the vehicle occupant may be impaired if it is too low.
It has been previously proposed to raise the front end of the seat only in case of an impact. Such proposals include those using an air bag (Japanese patent laid-open (kokai) publications No. 5-229378, No. 7-81466, and No. 3-227745), those which mechanically raise the front end of the seat (Japanese UM laid-open (kokai) publications No. 2-149328, No. 3-121947, and No. 4-93222), and those using a pyrotechnic actuator to mechanically raise the front end of the seat (Japanese UM laid-open (kokai) publication No. 3-61446).
However, because such anti-submarine vehicle seat devices involve an instantaneous generation of a large force, it is necessary to ensure a high positional accuracy of various component parts and a high level of mechanical strength. These factors contribute to increasing the complexity of assembling the seat assembly. It is also desired that the anti-submarine vehicle seat device may be retrofitted to existing seats.